The usefulness of the RF or microwave field application for monitoring of liquid level is recognized by the prior art. Such devices can operate with either RF or microwave excitation. When an electromagnetic field is excited in the container partially filed with liquid, parameters of the electromagnetic field, such as resonant frequency, vary with the level of the liquid. In particular the state of the art is shown in V.A. Viktorov "Microwave Method of Level Measurement", The Resonance Method of the Level Measurement, Moscow: Energija. 1987, disclosing an electrodynamic element, made as section of a long line, inserted into a monitored container where the resonant frequency is measured.
A general discussion, see Viktorov V. A., Lunkin B. V., Sovlukov A. S. "Method of and Apparatus for Level Measurement by Hybrid Electromagnetic Oscillation Excitation", Radio-Wave Measurements, Moscow: Energoatomizdat, 1989, states that an electrodynamic element is placed in a monitored container, and the element's resonance frequency has a direct correlation to the level of the liquid within the container.
Slowed electromagnetic waves and slow-wave structures are also well known in the field of microwave engineering, see J. R. Pierce, "Traveling-Wave Tubes" D. Van Nostrand Company, Inc., Princeton, N.J., 1950. These waves are electromagnetic waves propagating in one direction with a phase velocity v.sub.p that is smaller than the light velocity c in a vacuum. The relation c / v.sub.p is named slowing or deceleration and is designated as n. In the most practically interesting cases, slowed electromagnetic waves are formed in slow-wave structures by coiling one or two conductors (for example, into a helix, as it is shown in FIG. 1 (prior art), where the other conductor is a cylinder), which increases the path length traveled by the wave, or by successively connecting resonant elements or cells, energy exchange between which delays the phase of the wave, or by using an electrodynamically dense medium (usually a dielectric), or a combination of these methods. Additional deceleration was also obtained due to positive electric and magnetic coupling in coupled slow-wave structures, see V. V. Annenkov, Yu. N. Pchelnikov "Sensitive Elements Based on Slow-Wave Structures" Measurement Techniques, Vol 38, #12, 1995, pp. 1369-1375.
The slow-wave structure-based sensitive elements are known in the art, see Yu. N. Pchelnikov, I. A. Uvarov and S. I. Ryabtsev, "Instrument for detecting Bubbles in a Flowing Liquid", Measurement Techniques, Vol 22, #5, 1995, pp. 559-560, and Yu. N. Pchelnikov, "Possibility of Using a Cylindrical Helix to Monitor the Continuity of Media", Measurement Techniques, Vol. 38, #10, 1995, pp. 1182-1184. The slowing of the electromagnetic wave leads to a reduction in the resonant dimensions of the sensitive elements and this enables one, by using the advantages of electrodynamic structures, to operate at relatively low frequencies, which are more convenient for generation and are more convenient for primary conversion of the information signal, but sufficiently large to provide high accuracy and high speed of response. The low electromagnetic losses at relatively low frequencies (a few to tens of megahertz) also helps to increase the accuracy and sensitivity of the measurements. The slowing of the electromagnetic wave leads also to energy concentration in the transverse and longitudinal directions, that results in an increase in sensitivity, proportional to the slowing down factor n. See V. V. Annenkov, Yu. N. Pchelnikov "Sensitive Elements Based on Slow-Wave Structures" Measurement Techniques, Vol 38, #12, 1995, pp. 1369-1375.
Most slow-wave structures were made as two-conductor periodic transmission lines (see Dean A. Watkins "Topics in Electromagnetic Theory", John Willy & Sons, Inc. Publishers). A version is possible when a slow-wave structure contains three or more different conductors. In all cases the slowed wave is excited in the electrodynamic element between different combinations of the two conductors. The coiled conductors increasing the wave path are named "impedance conductors", and conductors with simple configuration such as rods, tapes, etc., stretched along the wave propagation direction are named "screen conductors", see V. V. Annenkov, Yu. N. Pchelnikov "Sensitive Elements Based on Slow-Wave Structures" Measurement Techniques, Vol 38, #12, 1995, pp. 1369-1375.
Both the prior art and the present invention measure one or more parameters of an electromagnetic field. Some of the prior art methods and present invention use an electrodynamic element, some are made as a resonant cavity filled with measured liquid or made as an electrodynamic element placed in or outside a container. The electrodynamic element is connected to an external RF or microwave signal generator which is used to excite an electromagnetic field. The change in, for example, the level of the liquid, causes a shift in the characteristics of the electromagnetic field in the electrodynamic element. The shift in characteristics correlates to a change, for example, in the level of the liquid within the measured container.
Devices used in the prior art exhibit several problems overcome by the present invention. Previous methods depend upon the sensitivity of a measured parameter of an electromagnetic field to measure level displacement and provide signal resolution. Sensitivity and resolution increase with frequency. However, the increase in frequency is accompanied by an increase in electromagnetic losses, such losses causing a loss of accuracy of the measurement. Besides, it is known that the higher frequency is, the higher is the cost of an electronics. The relatively low accuracy realized from the prior art is also due to resonant frequency dependence on the monitored liquid's electric parameters. Thus, there is a need in the art for an electromagnetic method and apparatus for monitoring liquid levels and other heights measurements that has better sensitivity, better resolution, greater diversity and lower cost.